Organic EL elements are broadly classified into low weight molecular type organic EL (OLED for short hereinafter) elements and polymer type organic EL (PLED for short hereinafter) elements.
It has been reported that an OLED element is improved in initial characteristic properties (such as low drive voltage and high luminous efficiency) and life if it is provided with a copper phthalocyanine (CuPC) layer as the hole injection layer. (Refer to Non-Patent Document 1: Applied Physics Letters, US, 1996, vol. 69, pp. 2160-2162)
It has been reported that a PLED element is also improved as in the case of OLED mentioned above if it is provided with a hole transporting layer (buffer layer) made from a polyaniline type material or a polythiophene type material. (For the former, refer to Non-Patent Document 2: Nature, UK, 1992, vol. 357, pp. 477-479, and Non-Patent Document 3: Applied Physics Letters, US, 1994, vol. 64, pp. 1245-1247. For the latter, refer to Non-Patent Document 4: Applied Physics Letters, US, 1998, vol. 72, pp. 2660-2662)
It has been reported that an organic EL element is improved in initial characteristic properties if it is provided (at its cathode side) with an electron injection layer made from any of the following materials. Metal oxide (refer to Non-Patent Document 5: IEEE Transaction on Electron Devices, US, 1997, vol. 44, pp. 1245-1248); metal halide (refer to Non-Patent Document 6: Applied Physics Letters, US, 1997, vol. 70, pp. 152-154); metal complex (refer to Non-Patent Document 7: Japanese Journal of Applied Physics, 1999, vol. 38, pp. 1348-1350)
The hole injection material for OLED elements is mostly a material for deposition. The disadvantage of a material for deposition is that it should meet several requirements. For example, it should be an amorphous solid, it should be sublimable, it should have good heat resistance, and it should have an adequate ionization potential (Ip for short hereinafter). This disadvantage narrows the range of choice for materials for deposition. Another disadvantage of a material for deposition is difficulties with electrically doping. The disadvantage prevents a material for deposition from exhibiting a high charge transporting performance. The result is that the charge injection efficiency remains low. CuPC as a commonly used hole injection material has a highly irregular shape and adversely affects the characteristic properties even when a very small quantity of it enters the other organic layer.
The hole transporting material for PLED elements should meet such requirements as high charge transporting performance, insolubility in a solvent for luminous polymer such as toluene, and adequate Ip. Polyaniline type material and polythiophene type material, which are commonly used at present, suffer several disadvantages as follows. They contain water as a solvent which might accelerate degradation of elements. They have a limited range of choice for solvents because of their low solubility. They are liable to aggregation. They are restricted in methods for forming uniform film.
Meanwhile, there has recently been found a charge transporting varnish (in the form of organic solution) which contains a low-molecular-weight oligoaniline type material as the charge-transporting substance. It has been reported that an organic EL element exhibits outstanding characteristic properties if it is provided with a hole injection layer made from the charge transporting varnish. (Refer to patent document 1: JP 2002-151272A.)
However, the charge-transporting varnish containing the low-molecular-weight charge-transporting substance (which may be incorporated with a charge accepting dopant substance) presents difficulties in forming a highly flat film. In addition, it usually has a low viscosity because the charge-transporting substance has a low molecular weight and hence a low viscosity. The varnish with a low viscosity presents difficulties in various coating process by printing, ink-jet spraying, and the like.
A common way to adjust the viscosity of varnish is by changing the molecular weight of the material or incorporating the material with a thickening agent.
However, change in the molecular weight of the charge-transporting material is accompanied by a great change in physical properties, such as charge transporting performance, Ip, solubility, and morphology. Thus it is difficult to adjust the viscosity without changing other physical properties. On the other hand, incorporation with a thickening agent tends to deteriorate the charge transporting performance.
For reasons mentioned above, it is difficult to adequately adjust the viscosity of the varnish while maintaining other properties such as charge transporting performance.
The present invention was completed in view of the foregoing. It is an object of the present invention to provide an improved charge-transporting varnish and its products in the form of charge-transporting thin film, organic EL element, and solar cell. The above-mentioned charge-transporting varnish exhibits highly uniform film-forming performance even in the system containing a low-molecular-weight charge transporting substance and a charge accepting dopant substance. When used in an OLED element or a PLED element, it exhibits outstanding EL characteristics, such as low drive voltage, high luminous efficiency, and long life.